Most loudspeakers use a vibrating diaphragm to couple mechanical energy developed from an electrical current to the air to produce acoustical energy. The diaphragm is suspended about its perimeter and vibrates along the central axis of the diaphragm. Most diaphragms for loudspeakers are in the form of cones or variations on cones.
It has been known for many years that the sound waves produced at the forward side of the loudspeaker diaphragm and the sound waves produced at the rearward side of the loudspeaker diaphragm are not in phase at all frequencies of the audible range. Accordingly, the sound waves produced at the rear side of the diaphragm, referred to as the back waves, interfere with sound produced at the forward side of the diaphragm. To eliminate or attenuate the back wave, loudspeakers are provided with baffles, the combination of the loudspeaker and baffle being known as a loudspeaker system. Loudspeaker baffles are preferably in the form of a hollow box, or enclosure, the enclosure having an opening in a wall to accommodatee the loudspeaker. Enclosures are sealed against air leakage except for the opening for the loudspeaker and in some cases a vent for acoustical purposes. Hence, the interior of the enclosure is subjected to the varying sound pressures produced by the back wave of the loudspeaker.
Ideally, a loudspeaker system should not color the sound in transforming electrical energy into acoustical energy. In order to avoid accentuating one frequency over another, resonances within the audible range must be avoided, whether those resonances are in the loudspeaker diaphragm or the enclosure. The walls of the loudspeaker enclosure are resonant at some frequency determined by the effective mass and stiffness of the wall. Even though the resonance may be damped, it is preferable that the mechanical resonance of the enclosure wall fall outside of the response range of the loudspeaker. The resonant frequency of the enclosure wall will be increased by descreasing the mass or increasing the effective stiffness of the wall. Hence, a wall with sufficiently low mass and sufficiently high stiffness can be made to have a resonant frequency above the response range of the loudspeaker. For those enclosure walls having a mechanical resonance in the response range of the loudspeaker, high stiffness and low mass will reduce the magnitude of the resonance.
A loudspeaker enclosure can also color the audio energy produced by generating sounds in the walls of the enclosure. A void in the wall of the enclosure will have at least one resonant frequency, and at that frequency, the void will cause the adjacent portions of the wall of the enclosure to vibrate excessively, thus creating a buzz or whistle which colors the acoustical energy produced. Hence, voids should be avoided in the walls of the enclosure.
Historically, wood has been the most popular material for loudspeaker enclosures. Wood is readily available, and in the past has been a relatively cheap construction material. Wood however is subject to nonuniformity, that is, the acoustical characteristics of wood vary with the type of wood and even with the particular piece of wood within a given category. Wood tends to have knots, and the knots tend to come loose. A loose knot will vibrate, and other sounds can be generated by voids. The stiffness and mass of wood will vary between different pieces of wood of the same general characteristics. Wood also has become costly in recent years.
Plywood has also been used in loudspeaker enclosures, often augmented by a vinyl wrap to improve the appearance of the enclosure. Plywood generally has at least three layers of wood, each layer having a grain which is oriented perpendicular to the adjacent layers. Plywood aften has voids, and often has an irregular outer surface making it more difficult to produce a loudspeaker enclosure which is attractive, either by treating the plywood itself, or cementing a vinyl wrap on the plywood. Voids in plywood are particularly objectionable in that they produce coloring sounds. Additionally, plywood tends to delaminate when subjected to moisture or temperature change.
More recently, particle board has been used for loudspeaker enclosures. Particle board comprises small generally uniform particles of wood or other fibrous materials in a plastic binder. Particle board generally has a high ratio of binder to wood particles, and the stiffness to weight ratio of particle board is lower than that of plywood. Accordingly, particle board results in an enclosure which is heavier than that of plywood, assuming the same wall stiffness.
Cast metal enclosures have also been used for loudspeakers, but these tend to be used only for very small loudspeakers in view of the cost of such structures. They are difficult to mold, and in addition, they generally utilize very thin walls and depend upon damping to avoid mechanical resonances.